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Mosfet Testing using a DVM

This video demonstrates a quick and simple way to check whether a MOSFET will turn on or off using two digital voltmeters (DVMs). You can use one DVM set to resistance (Ω) mode to measure the resistance between the Drain and Source. A second DVM is set to diode mode, which typically outputs about 1 mA at approximately 3 V. That small current is often sufficient to exceed the MOSFET’s gate-to-source threshold voltage (VGS(th)), allowing the device to turn on. When the diode tester is connected between the Gate and Source, it charges the gate capacitance. Once the gate voltage rises above threshold, a conductive channel forms and the Drain-to-Source resistance drops. An important point demonstrated in the video is that once the MOSFET turns on, it may remain on if there is no gate-to-source resistor present to discharge the stored gate charge. Because the MOSFET gate is insulated and behaves like a capacitor, the charge can remain trapped, keeping the device in the on state until the gate is discharged. This is a simple and effective way to illustrate MOSFET gate behavior and the importance of a proper gate pull-down resistor in practical designs. Questions or comments can be sent to: rbola35618@aol.com